Both CRM leads and contacts automatically sync to Marketo, out of the box (just note, even if the lead already exists in Marketo, it will not be de-duped against email address and a duplicate lead will be created). Using the custom sync filter, you can prevent this from happening based on specific scenarios (e.g., if no email address exists).

I would suggest checking to make sure you have the latest version of the Marketo connector installed in MS Dynamics. If you check "Step 2" in this tip you can see what version you have installed. In Marketo you can download the latest version of the connector and Sales Insights for Dynamics from the MS Dynamics area of the Admin area here:

It is a little challenging to see what the actual version is that you have downloaded, but if you follow the steps here you can find that out. For Dynamics 2016/365 the latest versions ate 4.0.0.21 for the connector and for Sales Insights is it 2.0.0.11 so if you are running 2016 or 365 make sure that your installed solutions are those versions. If not, you will need to update the installed solutions in DCRM.

Let me know what you find! In the event that your connector is up to date I would suggest it is time to open a case with Marketo support.